Session Initiation Protocol (SIP) is an application-layer control (i.e., signaling) protocol for creating, modifying, and terminating sessions with one or more participants. These sessions may include Internet-based telephone calls, multimedia distribution, multimedia conferences, instant messaging conferences, interactive voice response (IVR), automated and manual operator services, automatic call distribution, call routing, etc. SIP invitations or INVITES may be used to create sessions and may carry session descriptions that allow participants to agree on a set of compatible media types. SIP may use proxy servers to help route requests to a user's current location, authenticate and authorize users for services, implement provider call-routing policies, and/or provide other features to users. SIP may also provide a registration function that allows users to upload their current locations for use by proxy servers.
Networks implementing Voice over Internet Protocol (VoIP) may use network perimeter protection devices, such as firewalls, to block unwanted and/or potentially malicious traffic from infiltrating the network. Typical network perimeter protection devices fail to cope with the complexity of VoIP protocols at carrier-class performance.